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GNSS and PNT are acronyms that often seem to be used interchangeably when talking about satellite positioning and navigation. But they’re not the same, and as location-based technologies make ever-greater inroads into all of our lives, it’s worth understanding the differences between them.
What is GNSS?
Let’s start with GNSS. As an acronym, it means Global Navigation Satellite System. Back in the 1970s when the US Global Positioning System (GPS) was the only global navigation satellite system in the sky, there was no need for a further umbrella term to describe its function.
But over the years, GPS has been joined in orbit by three other GNSS. Russia’s GLONASS system became operational in 1985, Europe’s Galileo in 2014, and China’s BeiDou-3 in 2018. You may still hear ‘GPS’ used as shorthand to mean any global system of navigation satellites, but there’s growing recognition now that GNSS is the correct term to describe this type of service.
Features that all GNSS have in common
While there are differences between them, the four established GNSS systems have several features in common.
For a start, they all provide the same fundamental service. The signals broadcast by their satellites contain data that allows a receiver to calculate its position, navigate from one waypoint to another, and timestamp events to a fine degree of precision (usually plus or minus 10 nanoseconds).
All four GNSS systems offer freely-available signals for civilian (consumer and commercial) use, as well as encrypted, high-accuracy signals for governmental authorized users and sensitive applications. The civilian signals are used by receivers embedded in a huge range of devices—from smartphones and fitness trackers to cars, trains, ships, planes and drones, while the encrypted signals are used to co-ordinate nation-state activities and protect critical functions.
Static infrastructure like electricity substations and cell towers also make use of GNSS signals for the precise timing data they provide, which is important for synchronizing operations across energy grids and wireless communications networks.
Each GNSS has a constellation of 24+ operational medium Earth orbit (MEO) satellites, plus spares, deployed into Medium Earth Orbit (MEO), and all provide global coverage enabling a position to be calculated through a process of trilateration. In the case of China’s BeiDou-3, these MEO satellites are also supported by satellites in geostationary (GEO) and inclined geosynchronous orbits (IGSO).
What is PNT? How does it differ from GNSS?
PNT stands for positioning, navigation and timing, and is the term used to describe any technology, service or system that’s designed to enable positioning, navigation and timing capabilities in the full range of relevant applications.
GNSS is therefore one example—indeed the classic example—of a PNT service. However, GNSS isn’t the only service that supports positioning, navigation and precise timing. In many PNT systems today, a GNSS receiver is just one of an array of sensors and services that together enable the kind of precise, accurate, robust and resilient positioning required by equipment like autonomous vehicles.
Non-GNSS PNT sensors and services
PNT sensors and services that can complement and augment GNSS include:
Satellite-based augmentation systems (SBAS), ground-based augmentation systems (GBAS), and regional navigation systems: A wide array of services exist which use additional signals, data and algorithms to increase the accuracy and precision of GNSS-derived positioning. These range from regional satellite navigation systems like India’s NavIC and Europe’s EGNOS, to ground-based systems like Real-Time Kinematic (RTK) and Precise Point Positioning (PPP).
Inertial measurement units (IMUs): Also known as dead reckoning sensors, these include gyroscopes, accelerometers and altimeters. They deliver a relative positioning service, enabling a PNT system to continue to calculate a position when connection to a global positioning system (such as GNSS) is lost. However, drift in these systems means they require calibration by a precise global positioning system at regular intervals.
Oscillators: Crystal oscillators are embedded timing components which can maintain precise time for a certain period, but which start to drift if not regularly synchronized with a source of precise time such as GNSS.
Cameras: Camera vision is used in autonomous vehicles to detect obstacles, with the PNT system correlating the data with input from other sensors to calculate its position in relation to the vehicle.
Light Detection and Ranging (LiDAR): Similar to camera vision, LiDAR sensors can detect objects in the vicinity of the vehicle—data which can be used to calculate how far the vehicle is from the object and where the object is in relation to the vehicle.
Wi-Fi and cellular: Radio signals from known Wi-Fi access points and cellular (4G and 5G) towers can be used to establish position to a reasonable degree of accuracy. Wi-Fi positioning is particularly useful in indoor locations where the weak signals from GNSS often can’t penetrate.
Testing GNSS and PNT systems with Spirent
Spirent is the global leader in test and measurement for GNSS and additional PNT systems. Learn more about our test and measurement solutions for positioning, navigation and timing.
